Nowadays, steel sheets which are subjected to a surface treatment and provided with a rust prevention property, in particular, galvanized steel sheets or galvannealed steel sheets having excellent rust prevention property, are used as base steel sheets in the fields of automobile, domestic electrical appliance, and building material industries. In addition, the application of high-strength steel sheets to automobiles is being promoted in order to achieve weight reduction and strengthening of automobile bodies by decreasing the thickness of the materials of automobile bodies by increasing the strength of the materials from the viewpoint of an increase in the fuel efficiency of automobiles and the collision safety of automobiles.
In general, a galvanized steel sheet uses a steel sheet as a base material. The steel sheet is produced by hot-rolling a slab and cold-rolling the hot rolled steel sheet. The galvanized steel sheet is manufactured by performing recrystallization annealing on the base steel sheet in an annealing furnace used in a continuous galvanizing line (hereinafter, simply referred to as CGL), and by thereafter galvanizing the annealed steel sheet. In addition, a galvannealed steel sheet is manufactured by further performing an alloying treatment on the galvanized steel sheet.
It is effective to add Si and Mn in order to increase the strength of a steel sheet. However, in continuous annealing, Si and Mn oxidize and form oxides of Si and Mn on the outermost surface of the steel sheet even in a reducing atmosphere of N2+H2 in which oxidation of Fe does not occur (that is, oxidized Fe is reduced). Since oxides of Si and Mn decrease wettability between molten zinc and a base steel sheet when a coating treatment is performed, non-plating frequently occurs in a steel sheet to which Si and Mn have been added. In addition, even if non-plating does not occur, there is a problem in that coating adhesiveness is poor.
As described above, it is effective to add solid solution strengthening elements such as Si and Mn in order to increase the strength of a steel sheet. However, since oxides of Si and Mn are formed on the surface of a steel sheet in an annealing process, it is difficult to achieve sufficient adhesiveness between the steel sheet and the coating layer. Therefore, it is effective to first form a coating composed of iron oxides on the surface of a steel sheet by oxidizing the steel sheet and then to perform reduction annealing on the oxidized steel sheet.
As an example of a method for manufacturing a galvanized steel sheet whose base material is a high-strength steel sheet containing a large amount of Si, Patent Literature 1 discloses a method in which reduction annealing is performed after an oxide layer has been formed on the surface of the steel sheet. However, in the case of Patent Literature 1, it is not possible to stably realize the effect. Patent Literature 2 to Patent Literature 9 disclose techniques for stabilizing the effect, by specifying an oxidation rate and the degree of reduction or by controlling an oxidation condition and a reduction condition in accordance with the observation result of the thickness of an oxide layer which has been obtained in an oxidation zone.